The present invention generally relates to tundish vessels and, more particularly, to tundish impact pads designed to inhibit turbulent flow of molten metal within the tundish.
Tundishes are used to hold a quantity or bath of molten metal, such as molten iron or steel, delivered from a ladle through a ladle shroud. A tundish is disposed between the ladle and the casting apparatus or mold which receives the molten metal and forms various shaped products therefrom. The ladle is positioned several feet above the tundish and a ladle shroud, in the form of a long tube, leads from the ladle into the tundish. The ladle shroud delivers the molten metal to the tundish in a tight, compact stream. This incoming stream of molten metal can, for example, have kinetic energy ranging from 2 to 10 Watts/ton.
Pouring pads placed within tundishes have been widely used to prevent damage to the working and safety linings of a tundish by the force of the incoming stream of molten metal. The kinetic energy of the incoming stream of molten metal also creates turbulence which can spread throughout the tundish if the flow of molten metal is not properly controlled. Many times, this turbulence has a detrimental effect on the quality of cast products formed from metal taken from the tundish. More specifically, turbulent flow and high velocity flow within the tundish can, for example, have the following harmful effects:
1. Excessive turbulence can disturb the steel surface and promote emulsification of the slag at ladle changes or during operation of the tundish with a relatively low level of molten metal.
2. High velocities produced by turbulent flow in the pouring area can cause erosion of the working lining of the tundish which is typically comprised of a refractory material having a much lower density than impact pads.
3. Highly turbulent flow within the tundish can impede the separation of inclusions, especially inclusions less than 50 microns in size, due to the fluctuating nature of such turbulent flows.
4. High speed flows may also increase the possibility of slag being directed into a mold through increased vortexing of the molten metal in the tundish which draws slag downwardly toward the outlet.
5. Turbulent flow within the tundish may result in disturbance of the slag/metal interface near the top of the metal bath and thereby promote slag entrainment as well as the possibility of opening up an "eye" or space within the slag layer which can be a source of reoxidation of the molten metal.
6. High levels of turbulence in the tundish can be carried down into the pouring stream between the tundish and the mold. This can cause "bugging" and "flaring" of the pouring stream which thereby lead to casting difficulties.
7. High velocity flow in the tundish has also been attributed to a condition known as "short circuiting". Short circuiting refers to the short path a stream of molten metal may take from the ladle to the impact pad to the nearest outlet in the tundish. This is undesirable since it reduces the amount of time inclusions have to be dissipated within the bath. Instead, the high velocity flow sweeps relatively large inclusions down into the mold where they reduce the quality of the cast products.
A typical flat impact pad causes an incoming ladle stream to impact the top of the pad and travel quickly to the side or end walls of the tundish. When the stream reaches the side and/or end walls, it rebounds upward to the surface of the tundish where it changes direction toward the center of the tundish or, in other words, toward the incoming ladle stream. This creates undesirable inwardly directed circular flows in the tundish. The opposing flows on either side or end of the tundish travel toward the center of the tundish and carry with them slag or other impurities that have floated to the surface of the bath within the tundish. As a result, these impurities are drawn toward the incoming ladle stream and are then forced downwardly into the bath and toward the outlet or outlets of the tundish. This tends to cause more of these impurities to exit the tundish into the molds thereby decreasing the quality of the products produced within the molds.
While numerous other types of tundish pads have been proposed and used in the past, none of these fully address all of the problems noted above. Examples of prior tundish pads are disclosed in U.S. Pat. Nos. 5,131,635 and 5,133,535 both issued to Soofi and U.S. Pat. No. 5,169,591 to Schmidt et al. The tundish pads disclosed in the above patents to Soofi and Schmidt et al., however, are inadequate solutions to the above-mentioned problems at least because of the fact that they each direct the incoming ladle stream too directly toward the drain or drains of the tundish. Also, these pads do not slow the incoming ladle stream enough to completely address the problems associated with high velocity flows as mentioned above. In this regard, since each of these patents disclose impact pads which direct the incoming stream in either one or two lateral directions toward the drain or drains of the tundish, the speed of the ladle stream is not reduced enough to prevent many of the problems mentioned above. Moreover, directing the ladle stream toward the drain or drains of the tundish, as taught by these patents, leads to the previously explained problems of "bugging", "flaring" and "short circuiting".